Support for QSettings — PyQt 4.12 Reference Guide

# Support for QSettings

New in version v4.8.3.

Qt provies the `QSettings` class as a platform independent API for the persistent storage and retrieval of application settings. Settings are retrieved using the `QSettings.value()` method which, when using v1 of PyQt4’s `QVariant` API returns a `QVariant` object. An application will then typically convert the `QVariant` to the appropriate fundamental type using `QVariant.toBool()`, `QVariant.toInt()`.

The explicit use of these conversion methods works around a problem with the way different platforms implement the storage of settings. Some platforms only ever store string values which means that the type of the original value is lost. Explicitly calling a conversion method provides the missing type information about the value being retrieved.

A problem then arises when v2 of the `QVariant` API is used (i.e. the default for Python v3) because those explicit conversion methods are not available. Therefore a setting with an integer value of `42` may be retrieved as a string value of `'42'`. This inconsistency is made worse by being platform specific.

As a solution to the problem PyQt4’s implementation of `QSettings.value()` takes an optional third argument called `type`. This is either a Python type object, e.g. `int`, or a string that is the name of a C++ type, e.g. `'QStringList'`. The value returned will be an object of the requested type.

For example:

    from PyQt4.QtCore import QSettings, QPoint

    settings = QSettings('foo', 'foo')

    settings.setValue('int_value', 42)
    settings.setValue('point_value', QPoint(10, 12))

    # This will write the setting to the platform specific storage.
    del settings

    settings = QSettings('foo', 'foo')

    int_value = settings.value('int_value', type=int)
    print("int_value: %s" % repr(int_value))

    point_value = settings.value('point_value', type=QPoint)
    print("point_value: %s" % repr(point_value))

When this is executed then the following will be displayed for all platforms:

    int_value: 42
    point_value: PyQt4.QtCore.QPoint(10, 20)

If the value of the setting is a container (corresponding to either `QVariantList`, `QVariantMap` or `QVariantHash`) then the type is applied to the contents of the container.

For example:

    from PyQt4.QtCore import QSettings

    settings = QSettings('foo', 'foo')

    settings.setValue('list_value', [1, 2, 3])
    settings.setValue('dict_value', {'one': 1, 'two': 2})

    # This will write the setting to the platform specific storage.
    del settings

    settings = QSettings('foo', 'foo')

    list_value = settings.value('list_value', type=int)
    print("list_value: %s" % repr(list_value))

    dict_value = settings.value('dict_value', type=int)
    print("dict_value: %s" % repr(dict_value))

When this is executed then the following will be displayed for all platforms:

    list_value: [1, 2, 3]
    dict_value: {'one': 1, 'two': 2}